The concave grating spectrograph

The spectrograph has only three optical elements: the entrance slit, a cylindrical lens to correct astigmatism, and a concave grating that both diffracts the light and images the spectrum. In our instrument, wavelengths increase from left to right in the focal plane. The position corresponding to each wavelength is given by

$$m\lambda = \lambda_0 + k x$$ (101)

Here, $m$ is the order of interference (1, 2, ...) and $k$ is the dispersion (Å/mm). At the left edge of the focal plane, the wavelength corresponds to blue-green light in the first order. The yellow and red doublets appear near the center. Infrared light is imaged on the far right in the first order, but you may notice visible light there too. For example, the invisible near-infrared (9000 Å) in the first order corresponds to blue (4500 Å) in the second order which you can see and record on film. All the lines of interest in this experiment are on the left side and appear in the first order. The spectrograph is shown in Figure 10.2.

Figure 10.2: The Bausch and Lomb 1.5 meter concave grating spectrograph is illuminated by a sodium lamp. Light passes through the slit and the vertical mask to a lens that corrects is imaged at the center of the plate, and blue-green light at the left in the first order of interference.